The intrepid Yohkoh spacecraft has been taking X-ray pictures of the Sun
for more than ten years, and is still going strong. More than six million
Yohkoh "X-rays" of the Sun are helping astronomers better understand our nearest
star. The Japanese-led international mission, launched August 30, 1991, from
Kagoshima Space Centre, Japan. Astronomers are celebrating Yohkoh's tenth
anniversary with a scientific conference September 17-20 in Kona, Hawaii to
discuss its latest discoveries.

Scientists from the UK have designed and built one of the four instruments
on the spacecraft - the Bragg Crystal Spectrometer - a high resolution spectrograph
for X-rays. This effort was led by Professor Len Culhane of University College
London's Mullard Space Science Laboratory (MSSL) with collaborators at the
Rutherford Appleton Laboratory and the US Naval Research Laboratory. Funded
by the Particle Physics and Astronomy Research Council (PPARC) this UK led
instrument has been used to probe the origins of solar flares - huge explosions
on the Sun, in order to determine and predict when and where they occur.

The Yohkoh team has unravelled the processes that drive these explosions.
It is due to the sudden release of magnetic energy, which results in energetic
particles bombarding the surface of the Sun, causing material to surge outwards
with velocities of several hundred km/s. Yohkoh has revolutionised our view
of how the Sun works and paved the way for strong UK involvement in later
missions such as the ESA/NASA Solar and Heliospheric Observatory (SOHO) and
the Japanese/UK/US Solar-B mission.

Professor Culhane, Principal Investigator for the Bragg Crystal Spectrometer
instrument and Council Member of PPARC said:

"As the first of the new wave of solar physics space missions in the
last decade of the twentieth century, Yohkoh has forever changed the way in
which we look at the Sun. Along with its absolutely fundamental achievements
in Solar Physics, the results from Yohkoh have produced the new discipline
of Space Weather, an activity in which the UK plays a leading role. Future
work in this field will allow us to understand key features of the Sun's impact
on humankind. Yohkoh's success is an enormous credit to the efforts of Japan's
Institute of Space and Astronautical Science and to the contributions of NASA
and PPARC."

The UK also plays an important role in hosting the Yohkoh Data Archive Centre
which provides access to data from the mission. The centre which is located
at MSSL is funded through PPARC and was established in 1994 for the benefit
of the UK solar physics community.

An image of the sun taken on 4th September 2001 by the Soft X-Ray Telescope
onboard Yohkoh spacecraft can be found on the PPARC web site www.pparc.ac.uk
. Alternatively please contact Mark Wells on 01793 442100 or email mark.wells@pparc.ac.uk

Further images and information can be found on the following
web sites:-

The Sun's corona, including information about how and where this multi-million
degree outer layer of the Sun's atmosphere is heated to temperatures up
to hundreds of times greater than the solar surface. Yohkoh also tracked
the dramatic year-to-year evolution of the corona.

The physics of solar flares, titanic explosions in the atmosphere of the
Sun caused by the violent release of magnetic energy. A typical solar flare
can release in less than one hour as much as 10,000 times the annual energy
consumption of the US. Yohkoh observations have helped astronomers understand
better than ever before how the Sun's magnetic fields are deformed and twisted,
broken and reconnected during flares; and how the electrified gas (plasma)
of the Sun's corona is heated to millions of degrees during flares.

The structures that produce ejections of material from the Sun, helping
astronomers to understand and begin to predict "space weather". Although
the prediction tools are still rather primitive, two items of note along
this line are the discoveries that certain structures on the Sun, namely
sigmoids and trans-equatorial interconnecting loops (TILs), are more likely
to be the sites of solar eruptions. The sigmoids - S-shaped regions seen
in coronal imagery - have been found to be more like to erupt than non-S-shaped
regions, and were discussed at a NASA Space Science Update in May 1999.
The TILs have recently received attention as another possible source of
mass ejections.

Yohkoh is the first spacecraft to continuously observe the Sun in X-rays
over an entire solar cycle, the roughly 11-year cycle in which the Sun goes
from a period of numerous intense storms and sunspots to a period of relative
calm and then back again. Additionally, the Yohkoh Soft X-ray Telescope (SXT)
carries the longest-operating Charge Coupled Device (CCD) camera in space.
After 10 years, the CCD camera Ð similar in operation to digital cameras now
popular worldwide Ð still functions perfectly after collecting more than 6
million images.

Yohkoh is a mission of Japan's Institute of Space and Astronautical Sciences
with the cooperation of the United States and the United Kingdom. The US part
is funded by NASA; it comprises the building of the SXT by Lockheed-Martin
Solar & Astrophysics Laboratory (LMSAL), under the leadership of Dr. Loren
Acton, US Principal Investigator for SXT. A consortium of organizations is
responsible for the science operations of SXT and the Yohkoh data analysis,
including LMSAL, Montana State University, Stanford University, and the University
of Hawaii.

The collaboration has been extremely fruitful, with more than 900 peer-reviewed
publications and 100 master's and doctoral theses to date. Yohkoh data are
freely available on-line for interested scientists worldwide, and are being
analyzed in many different countries, including China, Saudi Arabia, India,
Argentina, Brazil, Russia, Australia, most European countries, and Canada.

According to the latest projections, Yohkoh will stay in orbit until the
next solar maximum, around 2010. In the coming years, Yohkoh will closely
collaborate with the High Energy Solar Spectroscopic Imager (HESSI), an upcoming
NASA mission, providing crucial calibration data for its high-resolution hard
X-ray images. Solar-B is the Japanese follow-up mission, again with involvement
from the US and the UK. It will look at the Sun in soft X-rays, as Yohkoh
before, but it will also make very high-resolution images in visible light.